The invention will be described in the context of an Automatic Vehicle Identification (AVI) system capable of exchanging data codes between an interrogator and a transponder. The AVI field is but one environment in Which the inventive concepts described herein can be applied. Systems using batteryless transponders or transponders with batteries may be used for identifying or locating objects bearing the transponders such as cattle, luggage or other items. Further, a transponder might provide status information regarding the object on which it is located, such as a transponder born on a car door indicating whether that car door is open. Transponders utilized in the above recognition systems or others may be powered from batteries or from wireless radio frequency (RF) signals.
With respect to AVI systems, generally, the interrogator is provided in a toll booth of a toll road, parking garage or other limited access facility. The interrogator (reader) identifies passing automobiles by sending wireless interrogation signals to a transponder (tag), which would normally be a small, self-contained unit placed, for example, on the dashboard or windshield of the car. In this way the car (or other vehicle or object) can be identified in a speedy and efficient manner. Depending on the use of the system, an account associated with the driver, owner, or other designated person can be debited with an access charge. Compatibility standards for one such AVI system is set out in Title 21, Division 2, Chapter 16, Articles 1-4 of the California Code of Regulations, herein known as the Caltrans specification or Caltrans spec.
With respect to the specific embodiment, which is compatible with the Caltrans spec, the minimum role of the interrogator is to: 1) trigger or activate a transponder; 2) interrogate the transponder for specific information; and 3) provide an acknowledgement message to the transponder after a valid response to the interrogation has been received. The immediate mandate of the Caltrans spec covers electronic toll collection, sometimes described as a part of "Electronic Tolls and Traffic Management" (ETTM). The AVI equipment for toll collection will typically consist of two functional elements: vehicle-mounted transponders and fixed-position interrogators.
A toll collection site will consist of at least one interrogator operating in the role described above. Upon interrogating or "polling" the transponder for specific information such as a transponder identification (ID), the interrogator (or a separate computer) will typically check the transponder ID against a database of valid, non-delinquent accounts. If the transponder ID is valid and non-delinquent, the interrogator will send a signal to a gate mechanism, or a toll site computer operating such a gate mechanism, to allow the car to pass. Of course other enforcement means are possible that may allow for less interruption of traffic, such as allowing all cars to pass and identifying the auto carrying the transponder (or the rogue automobile carrying an inoperable transponder or no transponder at all) by other means and notifying an appropriate enforcement agency.
The interrogation signal and response signal comprise data codes. Caltrans spec has set forth definitions for data codes to be transmitted between the interrogator and the transponder. The data codes described below are derived from the Caltrans specification and are merely exemplary and are intended to be neither an exhaustive nor a mandatory list of codes for a general AVI system.
(a) Agency Code: This 16-bit code field identifies the Agency that has authority to conduct the transaction; PA1 (b) Error Detection Code: The error detection code may be CRC-CCITT-16, with a generator polynomial of X.sup.16 +X.sup.12 +X.sup.5 +1. This results in a 16-bit error detection code transmitted with each data message; PA1 (c) Header Code: The Header is generally the first field in each data message for either reader or transponder transmissions and consists of an 8-bit and a 4-bit word for a total of 12 bits. The Header provides a "selsyn" signal that may be used by a receiver within a transponder or interrogator to self-synchronize (selsyn) with the data being received from the interrogator or transponder, respectively. An exemplary selsyn signal might the binary and hexadecimal values: 10101010 and AA, respectively. PA1 (d) The Header Flag code provides for a unique, 4 bit Flag that is recognized by a transponder or interrogator decoder as the end of the Header with the data message to follow. The exemplary Flag signal has binary and hexadecimal values: 1100 and C respectively; PA1 (e) Interrogator ID Number: This 32-bit field is used to uniquely identify the interrogator conducting the transaction; PA1 (f) Transaction Record Type Code: This 16-bit code uniquely identifies a specific type of valid transaction between a reader and a transponder. This code uniquely defines the transponder message fields and functions permissible. By way of example, hexadecimal numbers 1 through 7FFF may be set aside for transponder message structures and 8000 through FFFF may be dedicated for reader-to-transponder message structures; PA1 (g) Transaction Status Code: Used to provide status information to the transponder; and PA1 (h) Transponder ID Number: This 32-bit code uniquely identifies which transponder is responding to a polling request or is being acknowledged.
Because the transponders typically either derive their operating power from a small battery, or from a received RF signal, the transponders are not normally active. The interrogator will transmit an RF trigger pulse to activate (turn-on) the transponders in approaching cars or other objects. The interrogator may transmit a number of RF trigger pulses at regular intervals to wake up any approaching transponders. Alternatively, the interrogator might send an RF trigger pulse in response to an external stimulus to the interrogator indicating that a transponder is approaching (e.g. light, heat, or magnetic sensors). After a time delay, the reader then will transmit an encoded signal, referred to as the Polling message or interrogation which, upon detection and decoding by the transponder, will provide initial information to the transponder as to which data blocks the transponder should transit.
In a described embodiment, the interrogator transmits an unmodulated continuous wave RF signal as an interrogation signal to the transponder while waiting for the transponder response signal. By analogy to acoustic signals, an unmodulated RF signal is similar to a constant or "pure" musical tone without any variation in amplitude or frequency. However, it should be mentioned that a signal could be considered "unmodulated" in amplitude even if varying in frequency and vice versa. The transponder response signal in this embodiment comes when the transponder backscatter modulates the continuous wave RF signal with information from the transponder. Following the acoustic analogy, backscatter modulation is similar to the phenomenon achieved by singing into a fan and listening to the resulting sound. Typically when a person sings, they control the variations or modulations of their voice. Similarly, an RF transmitter is generally able to modulate its signal. However, when a person sings into a fan, the blades of the fan will reflect the sound of the voice immediately back to the person when the blades pass immediately in front of his mouth. Thus, the singer hears a chopping sound superimposed on his voice. That "chopping" sound the singer hears is nothing more than the amplitude variation of the reflection of the sound of his voice. Similarly, the transponder can modulate (by amplitude or other means) the continuous wave RF signal received from the interrogator and this reflected signal will have modulations superimposed on it.